FIG. 1 depicts a schematic diagram of a wireless local-area network 100 in the prior art, which comprises: access point 101, stations 102-1 through 102-N, wherein N is a positive integer, and hosts 103-1 through 103-N, interconnected as shown. Each station 102-i, wherein i is a member of the set {1, 2, . . . N}, enables host 103-i (a device such as a notebook computer, personal digital assistant [PDA], tablet PC, etc.) to communicate wirelessly with other hosts in local-area network 100 via access point 101.
QoS traffic (i.e., delay and jitter sensitive applications, like voice and video streaming) receives special treatment on the channel of a wireless LAN through special protocols for medium access control. The new draft standard for 802.11 LANs under preparation by Task Group E, referred to as 802.11e, offers a choice of two protocols, EDCF—a distributed random access protocol—and EPCF—a centralized deterministic polling protocol.
Distributed Access Protocol: EDCF, the QoS-enhanced version of the existing 802.11 DCF protocol, is a distributed random access protocol that allows delay and jitter sensitive frames to be transmitted with higher priority than ‘best-effort’ frames (i.e., frames that are insensitive to delay or jitter). Frames in the higher priority access categories can access the channel or start countdown of their back off delay after waiting for a shorter idle time interval following a transmission on the channel. Upon collision, priority access categories will double their contention window, but the maximum size achieved may vary by access category. This allows higher priority frames to stop doubling their contention window size sooner than lower priority categories, thus affording another means of differentiation.
Centralized Polling Protocol: EPCF, the point-coordinated version of what is referred to in 802.1le as HCF, is a centralized deterministic polling protocol that treats delay/jitter sensitive traffic preferentially when granting opportunities for uplink transmission. In a centralized polling protocol, the Access Point (AP) sends polls to the clients granting them the opportunity to transmit. Since a node transmits only upon receiving a poll, transmission is contention-free. Such a protocol may waste channel time, however, if it generates polls to stations having no data to transmit.
The choice of a protocol to use is complex, as it depends among other things on the type of traffic generated in a WLAN cell and on the overlap of the coverage areas of co-channel APs. While a detailed appraisal of the advantages of each protocol is outside the scope of this paper, it is interesting to note that in situations where the majority of the traffic in a cell is QoS traffic, the preferential treatment afforded to QoS frames by either protocol is irrelevant, as there are no frames over which QoS frames would gain preferential treatment. The comparison must be based on delay and throughput. The question then becomes: “which protocol accommodates more simultaneous independent QoS traffic streams within acceptable delay/jitter limits”? It is easy to see that the centralized polling protocol does better because channel time is not lost to contention. It is expected that, in general, there will be a mix of clients in a cell, some capable of communication based on the EDCF MAC protocol only, while others are able to communicate through either MAC protocol. Some APs may not provide EPCF service.
Emergency voice calls, referred to in the U.S. as ‘911 calls’, are of special concern in wireless LANs. There are two issues that require attention: call setup and voice data transmission: Attention must thus be paid to the following: (a) 911 calls must be set up promptly and (b) the voice in 911 calls should be heard with clarity to enable emergency response. With respect to the former, transmission for signaling packets must rely exclusively on the distributed MAC protocol (like EDCF), as the request to be polled (in situations where polled access is available) relies also on EDCF, and negotiation of that request may not be completed until after call setup. With respect to the latter, voice packets must be afforded sufficient prioritization so that they are transmitted within acceptable QoS specifications, regardless of the traffic load on the WLAN. If the 911 call is not handled with a deterministic centralized polling MAC protocol (like HCF polling), special provisions must be made for the 911 voice packets, to ensure that they are transmitted promptly.